In general, a fiber reinforced composite material of high porosity such as a carbon fiber reinforced carbon composite material has been reduced in porosity by a method such as resin (pitch) impregnation, CVD (chemical vapor deposition), pressure impregnation carbonization or the line, as described in "Tanso Sen-i Sangyo" by Ken-ichi Morita, Kindai Henshu-sha, pp. 176-178. In such a method, however, infiltration or a matrix raw material into a base material for the composite material is so limited that the interior of the base material may remain highly porous because of the difficulty in infiltrating of the matrix material, although porosity of a part around the surface layer of the base material can be lowered by impregnation of the matrix material. While in a resin impregnation method it may be possible to use a resin of low viscosity for impregnation into the interior of the base material for the composite material, such a resin of low viscosity is so inferior in yield upon carbonization that it is impossible to lower the porosity of the base material as a whole.
Ceram. Eng. SCI. PROC. Vol. 6, pp. 694-706 (1985) discloses a technique of lowering the porosity, which has been recently watched with interest. According to the method disclosed in this literature, matricies are formed through CVI in pores of a porous fiber reinforced composite material having a matrix made of carbon or ceramics such as SiC, Si.sub.3 N.sub.4, SiO.sub.2, TiC, Al.sub.2 O.sub.3, B.sub.4 C, TiN, BN or the like, thereby lowering the porosity of the porous fiber reinforced composite material.
FIG. 1 is a sectional view illustrating an apparatus for producing a high density fiber reinforced composite material, which is disclosed in the above literature. This apparatus comprises a reaction tube 1 having a gas inlet port and a gas outlet port, and a heater 4 for heating the reaction tube 1. A porous fiber reinforced composite material 3 is held on the inner wall surface of the reaction tube 1 by a holder 2. A reactive gas and a carrier gas are introduced into the reaction tube 1 from the gas inlet port, passed through the porous fiber reinforced composite material 3, and discharged to the exterior from the gas outlet port.
FIG. 2 is a partially enlarged sectional view showing a process of forming matricies 6 in pores 5 of the porous fiber reinforced composite material 3. Methyltrichlorosilan gas (CH.sub.3 Cl.sub.3 Si) is employed as a reactive gas for forming the matricies 6, and gaseous hydrogen (H.sub.2) is employed as a carrier gas for adjusting the density of the reactive gas. In this case, the methyltrichlorosilan gas is decomposed into silicon carbide and hydrogen chloride in accordance with the following reaction formula, so that silicon carbide is deposited in pores 5 of the porous fiber reinforced composite material 3 to define the matricies 6, as shown in FIG. 2: EQU CH.sub.3 Cl.sub.3 Si+H.sub.2 .fwdarw.SiC+3HCl+H.sub.2
Such decomposition of the methyltrichlorosilan gas is mainly caused when molecules of the methyltrichlorosilan gas collide with the porous fiber reinforced composite material 3. In addition, the methyltrichlorosilan gas is also decomposed when molecules of the reactive gas collide with each other in the gaseous phase. Decomposition products formed in the gaseous phase float as soot grains 7, and adhere onto the surface of the porous fiber reinforced composite material 3. Consequently, openings of the pores 5 exposed on the surface of the porous fiber reinforced composite material 3 are covered with the soot grains 7, such that diameters thereof are reduced. Thus, decomposition of the reactive gas, which is caused by collision of the reactive gas and the porous fiber reinforced composite material 3, mainly takes place on the surface of the porous fiber reinforced composite material 3. In other words, the matricies 6 are formed only on the surface of the porous fiber reinforced composite material 3, and hardly formed in its interior.
In order to sufficiently form the matricies 6 in pores which are provided in the interior of the porous fiber reinforced composite material 3, it is necessary to intermittently scrape off a layer of the soot grains 7 adhering onto the surface of the porous fiber reinforced composite material 3. Such an excess step is unpreferable in view of production efficiency.